Prior to the start of Printed Circuit Board (PCB) manufacturing, engineers review
all drawings, specifications and files to ensure manufacturability. Any identified
concerns or issues or communicated back to the customer for clarification or correction.
Once complete and all issues or concerns are resolved, the first step of the manufacturing
process is to release material to etch the inner layer circuit patterns.
Patterning | Etching - Photoengraving
The inner layer copper coated laminated is coated with resist, photoengraved and
developed. Afterward, the unwanted copper is etched off, and the remaining photo
resist removed. This is considered a subtractive photoengraving process.
Lamination
PCBs are made up of multiple layers, consisting of several thin etched boards or
trace layers which are bonded together through lamination (via heat and pressure).
Once pressed or laminated together, the inner layer stack is drilled to add interconnections.
Drilling
Small holes or vias are precision drilled in the layers using numerically controlled
drilling (NCD) machines and CAD files (excellon files). These files define the size
and position of each via in the design.
Controlled depth holes or vias can be used to drill just one layer of the circuit
board rather than drilling through all the layers. This is accomplished by drilling
individual sheets or layers of the PCB prior to lamination. Vias which connect a
layer to the outside surface of the Printed Circuit Board are called “blind vias.”
Vias that only connect interior layers (not to an outside surface) are called “buried
vias.” Via or hole walls are copper plated to connect conductive layers of printed
circuit boards.
Metallization - Solder Plating | Solder Resist
To enable electronic components to be mounted or soldered, metal “pads” and “lands”
are required. Bare copper is not readily solderable, so it requires plating with
a solderable material. Historically, lead based tin has been used for plating laminate
surfaces. However, now due to environmental regulations newer materials are being
used such as nickel, gold, or tin to offer solderability and compliance with new
standards.
Areas that should not be soldered are covered with a material to resist soldering.
Solder resist refers the a polymer coating that acts as a mask, thereby preventing
solder from bridging traces and possibly creating short circuits to nearby component
leads.
Outer Layer Fabrication
The above pattern etching (photoengraving, drilling and metallization steps) are
also performed on the outer layer circuits. Following metallization of the outer
layers final finishing is performed. Generally this includes application of solder
mask, silk screen, testing and packaging.
Final Finishing
Solder Mask
The processed laminates are coated with solder mask and cured or developed usually
via UV curing.
Silk Screen
Silk screen may be added to apply information to the outside surfaces of a printed
circuit board. This usually includes component designator information and other
relevant information such as logos, part numbers or manufacturer’s identification
or specification information.
Testing
Unpopulated circuit boards are subjected to a bare board test where each circuit
connection is verified by sending a small current through each contact point. Testers
typically are “bed-of-nails” or “flying-probe type to make contact with the copper
lands or holes on one or both sides of the board.
Routing
NC routing machines or scoring machines are used to aid is separation of the circuits
from a manufacturing panel as specified by individual customer requirements prior
to final packaging and shipment.